leetcode2两数之和

 输入：l1 = [2,4,3], l2 = [5,6,4]
 输出：[7,0,8]
 解释：342 + 465 = 807.

 /**
  * Definition for singly-linked list.
  * struct ListNode {
  *     int val;
  *     ListNode *next;
  *     ListNode() : val(0), next(nullptr) {}
  *     ListNode(int x) : val(x), next(nullptr) {}
  *     ListNode(int x, ListNode *next) : val(x), next(next) {}
  * };
  */
 class Solution {
 public:
     ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
         ListNode*dummy=new ListNode();
         ListNode*cur=dummy;//用dummy作哨兵节点head=dummy->next
         int carry=0;
         while(l1||l2||carry){
             carry+=(l1?l1->val:0)+(l2?l2->val:0);//(l1?l1->val:0)当l1=nullptr很好的避免了空节点的干扰
             cur=cur->next=new ListNode(carry%10);//精简代码相当于
             //cur=cur->next;cur=new ListNode(carry%10);
             carry/=10;
             if(l1)l1=l1->next;
             if(l2)l2=l2->next;
         }
         return dummy->next;
     }
 };